Mobile high-lift loaders or trucks of the type having extendible and retractable booms are commonly used in construction sites or the like for lifting a load from the ground level to an elevated position, as when lifting mortar, building blocks, bricks or the like from the ground to an upper level of a building under construction. In more limited use, they are also used to move a load from ground level to a position below ground level. The boom is typically constructed of telescoping sections with a boom butt that is mounted in vertically stationary relationship with respect to a truck frame. The boom tip is typically elevated, retracted, and moved up and down by hydraulic means. The boom is moved angularly also by hydraulic means. The boom must be collapsible for over-the-road transportation of the vehicle from site to site. This limits the length of the boom and, accordingly, the maximum reach of the boom when performing a task.
The free end of the boom or boom tip is equipped with a load-handling assembly typically comprised as a forklift carriage carrying forktines. During boom rotation, it is usually desirable to keep the forktines level. This is done by operation of a hydraulic cylinder as the boom is pivoted. Generally a master-slave hydraulic motor combination is used. As the boom is elevated, one hydraulic cylinder follows the elevation of the boom and is followed by a second hydraulic cylinder which corrects the angular orientation of the load-handling member with respect to the tip of the boom. This results in the forklift or other load-handling member maintaining a level or other positional relationship.